1. Field of the Invention
The present invention relates to a lubricating oil composition. More particularly, it relates to a lubricating oil composition which is excellent in frictional characteristics and stability against oxidation, inhibits its change with the lapse of time, and is effectively utilizable as a lubricating oil for various kinds of equipment such as an automatic transmission, a continuously variable transmission, a gear of a tractor, a wet brake, a wet clutch and a power steering.
2. Description of Related Art
A lubricating oil which can be used to lubricate an automatic transmission, a continuously variable transmission, a gear of a tractor, a wet brake, a wet clutch, a power steering or the like is required to have good properties of frictional characteristics, oxidation stability, corrosion resistance, rust resistance and the like. Especially important requirements are that a ratio of a coefficient of static friction to a coefficient of kinematic friction by which the frictional characteristics are indicated is small and that a change in this ratio with the lapse of time is small.
Heretofore, as such a lubricating oil, there is known a lubricating oil having a high coefficient of static friction and good transmission torque. This kind of lubricating oil, however, is disadvantageous in that the frictional characteristics are not sufficiently satisfactory and a shift shock is large.
In recent years, with the miniaturization of cars and the abrupt increase in FF cars (front engine front wheel driven cars), the tendency of miniaturizing the automatic transmission and the like more and more increases. Such a miniaturization of the automatic transmission inconveniently makes a driver feel the shift shock more sensitively. Thus, in order to relieve the shift shock and to thereby make a car more comfortable to drive, it has been a technical subject to improve the frictional characteristics especially at an initial stage.
For the purpose of improving the frictional characteristics at the initial stage, there have been suggested a lubricating oil containing a friction modifier (Japanese Patent Application Laid-open Nos. 173097/1985 and 180000/1988) and a lubricating oil in which a base oil having a specific composition is used (Japanese Patent Application Laid-open No. 254196/1988). In these lubricating oils, however, the change with the lapse time in the frictional characteristics is large, and so they are not considered to have a sufficient performance.